For individuals with spinal pathologies, the development of spinal fixation devices represents a major medical breakthrough. Surgically implanted fixation systems are commonly used to correct a variety of back structure problems, including those which occur as a result of trauma or improper development during growth. These fixation systems typically include one or more stabilizing rods aligned in a desired orientation with respect to a patient's spine. Additionally, anchoring screws are inserted into the patient's spinal bones, and a series of connectors is used to rigidly link the rods and anchors.
A variety of designs exist, with each design addressing various aspects of the difficulties that arise when one re-shapes an individual's spine to follow a preferred curvature. Unfortunately, known spinal implant systems often correct one set of problems only to create new ones.
Common to spinal implant systems is the necessity for proper anchoring to the bone so as to provide support for the aforementioned components. While bone screws are commonly used for anchoring, they are limited in their positioning due to the design of component pieces. Numerous patents are directed to component design in order to accommodate the bone screw, yet few patents are directed to bone screws that will accommodate existing component design. In most bone screw designs accommodation is made for applying anti-torque to the bone screw as other components are connected to the bone screws. This preserves the critical bone-screw interface which has been set when the screw is turned into the bone.
For this and other reasons, screws located in bone structure typically use a specially designed clamp to attach to a component such as an alignment rod. A problem with specially designed clamps is that bone structure cannot be determined until the patient's bone is exposed causing the necessity of a large inventory of various sized clamps to be on hand during surgery, of which the surgeon must search to find the right combination. Even if a clamp combination is predicted, insertion of the screw may still require angular insertion due to muscle or tender nerve locations. The result is a bone screw which exerts unpredictable forces upon attachment to component connectors. Further, any movement of muscle and other tissue increases the difficulty of the operation and can be a major trauma to a person.
A conventional bone screw consists of a single shaft with a coarse thread at one end for threading into the bone and a machine thread at the other end for coupling to components. Another type of bone screw has a U-shaped top which acts as a saddle for attachment to an alignment rod. If the screw is placed incorrectly for any reason, the rod clamp must be made to accommodate the position.
A number of patents exist which demonstrate the reliance on the saddle type screw support and various designs to accommodate the problem.
U.S. Pat. No. 5,133,717 sets forth a sacral screw with a saddle support. Disclosed is the use of an auxiliary angled screw to provide the necessary support in placing the screw in an angular position for improved anchoring.
U.S. Pat. No. 5,129,900 sets forth an attachment screw and connector member that is adjustably fastened to an alignment rod. An oblong area provided within each connector member allows minute displacement of the alignment rod.
U.S. Pat. No. 4,887,595 discloses a screw that has a first externally threaded portion for engagement with the bone and a second externally threaded portion for engagement with a locking nut. The disclosure illustrates the use of a singular fixed shaft.
U.S. Pat. No. 4,946,458 discloses a screw which employs a spherical portion which is adapted to receive a locking pin so as to allow one portion of the screw to rotate around the spherical portion. A problem with the screw is the need for the locking pin and the inability of the base screw to accommodate a threaded extension bolt.
U.S. Pat. No. 5,002,542 discloses a screw clamp wherein two horizontally disposed sections are adapted to receive the head of a pedicle screw for use in combination with a hook which holds a support rod at an adjustable distance.
U.S. Pat. No. 4,854,304 discloses the use of a screw with a top portion that is adaptable for use with a specially designed alignment rod to permit compression as well as distraction.
U.S. Pat. No. 4,887,596 discloses a pedicle screw for use in coupling an alignment rod to the spine wherein the screw includes a clamp permitting adjustment of the angle between the alignment rod and the screw.
U.S. Pat. No. 4,836,196 discloses a screw with an upper portion design for threadingly engaging a semi-spherical cup for use with a specially designed alignment rod. The alignment rod having spaced apart covertures for receipt of a spherical disc allowing a support rod to be placed at angular positions.
U.S. Pat. No. 5,800,435 sets forth a modular spinal plate assembly for use with polyaxial pedicle screw implant devices. The device includes compressible components that cooperatively lock the device along included rails.
U.S. Pat. No. 5,591,166 discloses an orthopedic bone bolt and bone plate construction including a bone plate member and a collection of fasteners. At least one of the fasteners allows for multi-angle mounting configurations. The fasteners also include threaded portions configured to engage a patient's bone tissue.
U.S. Pat. No. 5,569,247 discloses a multi-angle fastener usable for connecting patient bone to other surgical implant components. The '247 device includes fastening bolts having spherical, multi-piece heads that allow for adjustment during installation of the device.
U.S. Pat. No. 5,716,357 discloses a spinal treatment and long bone fixation apparatus. The apparatus includes link members adapted to engage patient vertebrae. The link members may be attached in a chain-like fashion to connect bones in a non-linear arrangement. The apparatus also includes at least one multi-directional attachment member for joining the link members. This allows the apparatus to be used in forming a spinal implant fixation system.
Another type of spinal fixation system includes rigid screws that engage the posterior region of a patient's spine. The screws are adapted with rod-engaging free ends to engage a support rod that has been formed into a desired spine-curvature-correcting orientation. Clamping members are often used to lock the rod in place with respect to the screws. Instead of clamping members, other fixation systems, such as that disclosed in U.S. Pat. No. 5,129,900, employ connectors that join the support rods and anchoring screws. The connectors eliminate unwanted relative motion between the rod and the screws, thereby maintaining the patient's spine in a corrected orientation.
Unfortunately, although these so-called “rigid screw” fixation systems can alter the curvature of a patient's spine, they can also be difficult to install. In this type of system, the anchoring screws must be secured in a region that is strong/rigid enough to support the characteristically-large loads typically transferred from the support rods. As a result, the number of suitable anchoring locations is limited. Typically, these screws are anchored into the posterior region of a patient's spinal column or into pedicle bone. With rigid screw systems, installation requires bending a support rod into a path that will not only correct the shape a patient's spine but that will also engage each of the installed anchoring screws. Achieving a proper fit between all of the components while contending with the constraints encountered during surgery is often difficult. In severe cases, a suitable fit may not be achieved and the surgery will be unsuccessful.
Additionally, the nature of the installation process required for rigid screw fixation systems often subjects the system components to pre-loading that unduly stresses the interface between the patient's bone and the employed anchoring screws. With these designs, as a patient moves about during daily life, the system components may become separated from the supporting bone. Corrective surgery to reattach anchoring screws exposes an already-weakened region to additional trauma and presents the risk of additional damage.
Other spinal fixation systems employ adjustable components. For example, U.S. Pat. No. 5,549,608 includes anchoring screws that have pivoting free ends which attach to discrete rod-engaging couplers. As a result, the relative position of the anchoring screws and rods may be adjusted to achieve a proper fit, even after the screw has been anchored into a patient's spinal bone. This type of fixation system succeeds in easing the rod-and-screw-linking process. This adjustment capability allows the screws to accommodate several rod paths. Unfortunately, some adjustable fixation systems tolerate only limited amounts of relative adjustment between components, operating best when loaded in one of several preferred arrangements. As a result, many prior art adjustable fixation systems are suitable for only a few situations.
Additionally, many adjustable fixation systems are prone to post-surgery component loosening. As a patient moves about during day-to-day living, his spine is subjected to a seemingly-endless amount of dynamic loading. Almost all activity requires some form of back motion; over time, this cyclic movement tends to work the components of many adjustable fixation systems loose.
Some adjustable spinal fixation systems include locking mechanisms designed for long-term, post-surgery securement of the system components. Although capable of being locked in place, these systems are often difficult to secure, requiring an excess of tools during the installation process. The need for extra tools, such as those required to shave, to apply anti-torque, or crimp key portions of a fixation system, increasing surgical risk by adding complexity and increasing the number of required steps. Although locking-component fixation systems exist, many of them unduly increase the dangers of back implant surgery to an unacceptable level.
Hardware-intensive fasteners are disclosed in U.S. Pat. No. 5,549,608, in which anchoring screws are fitted with wrenching flats that allow an anchoring screw to be attached to a patient's spinal bone with the flats being trimmed away once the screw is in place. Clamping nuts are then used to secure the anchoring screws to included stabilizing rods.
Additionally, many spinal fixation systems do not permit component repairs. If, for example, a threaded portion of a connecting member becomes stripped or cross-threaded, the entire connector must be slid off of the associated stabilizing rod. Often, such removal produces an undesirable “domino-effect,” requiring that several connectors be slid off to allow removal of the damaged connector. Such requirements add unnecessary difficulty to an already-complex procedure.
The bone screws shown and described in U.S. Pat. Nos. 5,628,740 and 6,050,997 have a bone screw with a spherical cavity in the proximal end. A toggle bolt with a spherical distal end is inserted into the cavity in the bone screw. A collet is forced into the spherical cavity superior to the spherical end of the toggle bolt. A support collar or attachment cap is placed over the toggle bolt and tightened down. This forces the retention collet to engage the spherical portion of the toggle bolt and the inside of the spherical cavity locking the toggle bolt in a selected angular disposition. This system requires extremely accurate machining of the threaded components to result in an optimum frictional fit. Further, because the collet is a ring, with a fixed inner diameter, there is only one correct size for the spherical components. Finally, any deformation of the ring will lessen the over-all frictional contact by creating wrinkles or ridges on the collet.
U.S. Pat. No. 4,419,026 to Leto discloses a split ring camming internal locking device used with telescoping tubular members for transporting liquids. The ring is split for flexing to fit around the internal tube and for resiliently sealing against the external tube.
Thus, what is needed is a spinal fixation system that includes the advantages of known devices, while addressing the shortcomings they exhibit. The system should allow component adjustment during installation, thereby enabling satisfactory correction of a wide variety of spinal deformities. The system should also include a component locking mechanism that is simple and reliable. The system should also include mounting hardware that secures with a minimum of tools and that allows modular replacement of components damaged during installation. The system should also include tools and components for the locking mechanism developing a compression fit between components without additional torque on the bone-screw interface.